From the standpoint of global warming, there is a movement to limit the use of chlorofluorocarbon refrigerant, which has high global warming potential. Hence, railroad-vehicle air-conditioning devices that use carbon dioxide refrigerant, which has low global warming potential, have been proposed (see Patent Literature 1, for example). The technique disclosed in Patent Literature 1 relates to an air-conditioning device installed on the ceiling of a railroad-vehicle. The vehicle air-conditioning devices to be installed on the ceiling like this are mainly used in vehicles that are operated at about 130 km/h or less.
On the other hand, in high-speed trains (e.g., the Shinkansen trains and limited express trains), vehicle air-conditioning devices are usually installed under the floor. There is no underfloor vehicle air-conditioning device that uses carbon dioxide as the refrigerant.